A distribution which we will make use of later is the distribution of
time intervals between events from a random source. Suppose we observe
a radioactive source with a mean rate
. The probability of observing
no counts in a period T is then given by (57). In a manner similar
to Section 5.4, we can interpret (57) as the
probability density for
the time interval T during which no counts are observed. Normalizing
(57), we obtain the distribution

(60)

for the time T between counts. Equation (60) is just an exponential
distribution and can, in fact, be measured.